1. Field of the Invention
This invention relates to a method for the continuous electroplating of copper on workpieces to be plated by use of a copper sulfate plating bath.
2. Description of the Related Art
In the formation of patterns of printed circuit boards or wafers, copper sulfate electroplating is carried out. This copper sulfate plating bath containing organic additives called brightener, leveler, promoter, controlling agent and the like. In this connection, however, it is known that in the course of continuous plating, these organic additives are decomposed or degenerated (a compound or compounds obtained after decomposition or degeneration may be sometimes called hereinafter decomposed/degenerated organic product or products), so that a desired copper plating film or copper plating deposition is not obtained. In order to avoid copper slime generated owing to the use of a phosphorus-containing copper anode from being incorporated into a plating film, a copper sulfate plating process has been adopted using an insoluble anode. Where continuous plating is carried out, not only there arises a problem on the above-mentioned decomposed/degenerated organic products, but also copper ions and organic additives in the plating bath are reduced in amount, for which it becomes necessary to control the missing copper ions and organic additives by replenishment.
In such a copper sulfate electroplating method, it is essential to avoid the problem on the above decomposed/degenerated organic products and also to continuously perform copper sulfate electroplating while replenishing plating components and keeping the characteristics of the plating film. Prior art technique of copper sulfate electroplating includes those indicated below.
Japanese Patent Laid-Open No. Hei 3-97887:
In this document, air agitation is carried out in a separate vessel provided with a copper metal in current-off condition so as to replenish copper ions. Since the supply of the copper ions and the decomposition of decomposed/degenerated organic products are carried out in the same vessel, so that exact controls of the maintenance of copper ion concentration and the oxidative decomposition of the decomposed/degenerated organic products are incompatible, thereby disenabling the characteristics of plating film to be maintained.
Japanese Patent Laid-Open No. 2003-55800:
Blank electrolysis is carried out in a separate vessel by use of an insoluble anode, and decomposed/degenerated organic products are reduced in amount by oxidative decomposition by means of oxygen generated from the insoluble anode. However, when plating is continuously performed, it takes too long a time to satisfactorily decompose the decomposed/degenerated organic products by oxidation, thus presenting a problem from a practical standpoint.
Japanese Patent Laid-Open No. 2003-166100:
This document describes a method in which iron ions are contained in a copper sulfate plating bath as a redox material, and copper power is added to the plating bath in a separate vessel. However, since iron ions are contained, the iron ions may be co-deposited in the resulting plating film and thus, the characteristics of the plating film cannot be maintained.
Japanese Patent Laid-Open No. 2004-143478:
Air agitation is carried out in a separate vessel so as to increase an amount of dissolved oxygen in a plating bath, in which decomposed/degenerated organic products are oxidatively decomposed. However, only the air agitation disenables the oxidative decomposition of the decomposed/degenerated organic products to proceed satisfactorily. Although it may be possible to make the air agitation strong, stronger air agitation leads to larger-sized bubbles being returned to the plating vessel. When the large-sized bubbles are incorporated into the plating vessel, the bubbles attach to a workpiece being plated, thereby causing a plating failure such as nonplating.
Japanese Patent Laid-Open No. 2005-187869:
In a separate vessel, copper is provided in current-off condition and air agitation is carried out to control such organic additives as set out hereinabove. Simultaneously, the concentration of copper ions is held in another copper dissolution vessel, and the copper ions dissolved in the copper dissolution vessel are transferred to the separate vessel. In this case, in order to replenish the shortage of the copper ions, it is necessary to continuously return, to a plating vessel, a given amount of the plating bath in the copper dissolution vessel in correspondence with the consumption of the copper ions. In this condition, when the decomposed/degenerated organic products are accumulated, the plating bath is returned to the plating vessel even under conditions where the oxidative decomposition of the organic additives is not satisfactory. Accordingly, it is not possible to control both the concentration of copper ions and the oxidative decomposition of organic additives. Only one decomposition vessel for the oxidative decomposition of the decomposed/degenerated organic products is used, so that if the oxidative decomposition treatment is carried out under conditions of continuously circulating the plating bath, the plating bath has to be returned to the plating vessel before oxidative decomposition of the decomposed/degenerated organic products does not proceed satisfactorily. On the other hand, when the oxidative decomposition treatment is carried out in a batchwise manner, the solution level in the plating vessel differs between the case where the plating bath is filled in the decomposition vessel and the case where not filled, thereby causing a plating failure.